Electric power conversion systems have been used in motor vehicles, houses, plants, etc. Such electric power conversion systems convert an electric power of a power source to a predetermined electric power, for example, converts a direct current electric power to an alternating electric power.
Such a type of the electric power conversion system is comprised of an electric power conversion circuit, a capacitor, and an electrical switching means. The electric power conversion circuit has a series connection unit comprised of a switching element in a high voltage potential and a switching element in a low voltage potential which are connected in series. The electric power conversion circuit converts an electric power of a direct current power source to a predetermined electric power, for example, converts a direct current power to an alternating current power. The capacitor is placed between the electric power conversion circuit and the DC power source. The electrical switching means opens and closes electrical connections between the electric power conversion circuit, the capacitor and the direct current power source. When the electrical switching means is open, the discharge control device for the electric power conversion system controls the switching element at a high voltage potential and the switching element at a low voltage potential to discharge the voltage charged in the capacitor to a voltage of not more than a predetermined voltage.
For example, because an inverter (as an electric power conversion circuit), etc. connected to a motor generator as a vehicle main engine receives a very high voltage, the inverter, etc. are placed in an on-vehicle high voltage system. Such an on-vehicle high voltage system is electrically insulated from an on-vehicle low voltage system. The on-vehicle low voltage system is comprised generally of an on-vehicle control device, etc. A power source for supplying an electric power to a drive circuit for each of switching elements in the inverter is comprised of an insulation type converter. An input terminal of the insulation type converter is electrically connected to a battery placed in the on-vehicle low voltage system.
By the way, such a capacitor is generally connected to the input terminal of the inverter in order to smooth the voltage between the pair of the input terminals of the inverter. It is necessary to discharge the capacitor when the inverter is stopped.
For example, as disclosed by the following patent document 1, there has been proposed a technique for making a short circuit between both electrodes of a capacitor by turning on a switching element at a high voltage potential and a switching element at a low voltage potential simultaneously in order to discharge electric power charged in the capacitor. The capacitor is connected to a pair of input terminals of the inverter.